1. Field of the Invention
The present invention relates to a semiconductor optical component comprising a stack of layers, and more particularly it relates to a semiconductor laser.
2. Description of the Related Art
A semiconductor laser can be used as a pumping source for amplifying optical signals transported over long distances by an optical fiber. This is known in the art.
A semiconductor laser is an optical component, generally including a stack of doped or undoped semiconductor layers, and in particular an active layer. When fed with current, the active layer emits radiation by electroluminescence. From above a threshold current, the radiation emitted by the active layer can be amplified and then constitutes laser radiation at wavelengths in a band situated around 980 nm in the case of some optical pumping lasers, for example. The laser, which is generally in the shape of a rectangular block, can have front and rear laser lateral faces cleaved to form faceted mirrors so that Fabry-Perot longitudinal propagation modes are established in the laser.
Throughout the remainder of the description, the term “layer” may refer either to a single layer or to a stack of layers with the same function.
To be more precise, this type of laser conventionally comprises an n-doped substrate based on gallium arsenide (GaAs) on which are successively deposited:
an n-doped AlGaAs bottom cladding layer,
an InGaAsP bottom confinement layer,
an InGaAsP active layer forming one or more quantum wells,
an InGaAsP top confinement layer,
a p-doped AlGaAs top cladding layer, and
one or more contact layers, generally layers of GaAs, for making ohmic contacts, i.e. for correct flow of injected current to the active layer.
The confinement layers are also known as separate confinement heterostructures (SCH).
The active and confinement layers form the optically active area of the laser, which is also known as the light guide.
Researchers are currently seeking to increase the power coupled from a laser of the above kind into the interior of the optical fiber to which it is connected, for constant injected current.
For this purpose, it is known in the art to use a broad waveguide (BW) laser whose top and bottom confinement layers are much thicker than in conventional lasers. The thickness of the confinement layers in a BW laser is typically of the order of 600 nm, as against 100 nm for a conventional laser. This confines practically all (typically 99%) of the mode in the active layer, and thus increases the external efficiency of the laser.
However, increasing the thickness of the confinement layers significantly widens the light guide. This significantly increases the vertical divergence of the laser, because the mode is very “pinched” in the active layer, which causes high divergence at the output of the laser. Vertical divergence is the angle at the apex of the exit cone of the laser beam in a plane perpendicular to the plane of the semiconductor layers. It is typically of the order of 32°.
It is known in the art to place a device such as a lens at the output of the laser, before the fiber, which device is intended to narrow the laser beam in the vertical direction to solve this problem.
That solution is not satisfactory, however, and cannot be envisaged on an industrial scale, because the coupling ratio obtained is not reproducible.